In conventional MIMO (Multiple Input Multiple Output) systems, DL (downlink) precoding is usually performed on per sub-band which consists more consecutive subcarriers (such as consisting of 60 consecutive subcarriers), so DL reference signals can be inserted in each sub-band to facilitate the estimation of the DL effective channel. The size of the sub-band must be smaller than the correlation bandwidth of the channel, so that channels within one sub-band can be roughly regarded as flat. All subcarriers within one sub-band can use the same precoding matrix, which is calculated from the average CSI (Channel State Information) of this sub-band. Then using the same precoding matrix to perform precoding to reference signals, and the precoded reference signals are then inserted in each sub-band. At the user side, each user estimates the effective channel on a per sub-band basis using the DL reference signals.
The disadvantage of per sub-band precoding is the additional multiuser interference resulted from the mismatch between the precoding matrix (calculated from the average CSI) and the actual channel on each subcarrier. The advantage of per sub-band precoding is that it allows the insertion of reference signals to aid DL effective channel estimation. But, with the growth of antenna number, the advantage of per sub-band precoding shrinks and its disadvantage expands. So, in a massive MIMO system, the smaller precoding granularity is preferred in frequency, but since the overhead of reference signal increases with the reduce of the precoding granularity, it makes it hard to DL channel estimation.